*{
  margin: 0px;
  padding: 0px;
}
.header{
  margin: 0 auto;
  width: 1080px;
}
.on{
  margin-right: 10px;
  float: left;
  position: relative;
  width: 100px;
  height: 30px;
  background-color: bisque;
}
.in{
  position: absolute;
  top: -90px;
  width: 100px;
  height: 120px;
  z-index: -1;
}
.a{
  width: 100px;
  height: 30px;
  background-color: brown;
}